The present invention relates to medical devices and, more particularly, to vascular stents.
Stents are prosthetic devices which are implanted inside a lumen in order to provide support for and assure patency of the lumen. Patency is particularly important in the field of angioplasty which is concerned with the repair and reconstruction of blood vessels. Stents are frequently implanted within the vascular system to reinforce collapsing, partially occluded, weakened, or abnormally dilated sections of blood vessels. More generally, however, stents can also be used inside the lumen of any physiological conduit including the arteries, veins, bile ducts, urinary tract, alimentary tract, tracheobronchial tree, cerebral aqueduct, and genitourinary system. Stents can also be used inside lumina of animals besides humans.
One common procedure for implanting a stent in a blood vessel involves mounting the stent on a catheter. The catheter is then slipped through an incision in the vessel wall and down the length of the vessel until it is positioned to bridge a diseased or narrowed portion of the vessel. If the stent is expandable, it may be mounted on a balloon-tip catheter in its unexpanded form and then expanded in place against the inside wall of the vessel at the same time as the vessel is being dilated by the balloon.
Conventional stent designs have been found to have many shortcomings. For example, U.S. Pat. No. 4,553,545 to Maass et al. discloses a device for application in blood vessels including a helically shaped coil spring that can be expanded from a first state of a certain diameter to a second state of a larger diameter (and vice versa) by rotating the ends of the coil relative to each other. The length of the spring may be maintained during the transition from the first state to the second state by changing the number of spring turns and the corresponding pitch of the spring as the coil is wound. When the Maass design is applied to surgical operations in the human body, the expansion degree is limited to about fifty percent for an expansion number of 1.5.
The large size and limited expendability of the Maass design necessitates a relatively large incision in the vessel wall before it can be inserted with a catheter. The Maass device may, therefore cause excessive vessel damage and bleeding during implantation, especially when it is used inside large vessels such as arteries. Furthermore, changing the number of turns and pitch of the Maass coil a vessel may damage the vessel intima as the outside edges of each coil, and the ends of the stent, rub against the interior wall of the vessel. Moreover, unless the ends of the Maass device are translationally fixed during expansion of the coil, the ends of the coil may migrate along the axis of the stent and cause further damage to the inside of the lumen. Consequently, the Maass design is difficult to expand once it is in place inside the lumen.
Cesare Gianturco and Gary Roubin have developed the so-called "Gianturco-Roubin" stent illustrated in FIG. 1. This stent is constructed of monofilament stainless steel alloy wire 0.006 inches in diameter which is configured in the shape of an incomplete coil. The stent is balloon expandable and may be premounted on a polyethylene balloon catheter. The two-to-one expansion ratio of this stent limits this design to small vessel applications. Large vessels such as aorta would require a prohibitively large introduction system and therefore put the vessel at risk for excessive bleeding.
The present inventor has found that vascular stents require substantial flexibility in their unexpanded state in order to allow them to bend and conform to the tortuous shape of the vessels through which they are inserted. This need for flexibility during insertion is especially important for older patients since their blood vessels tend to be more tortuous and less flexible than those of younger patients. The present inventor has also found that, vascular stents should be rigid and have a high hoop strength in their expanded state. Although the reasons for the success of rigid stents are not entirely clear, it has been suggested that rigid stents are less likely to pulsate inside vessels, and therefore, they are less likely to rub against the vessel intima once they are in place. The Gianturco-Roubin stent suffers from the limitation that it is not particularly rigid in its expanded state. The openness of the one side of the Gianturco-Roubin coil design also results in an irregular lumenal appearance that may predispose the ingrowth of tissue through the coils. Overexpanding the stent may increase the size of this open portion and thus increase the possibility for excessive tissue ingrowth.
Gianturco has also disclosed the self-expanding "Z" stent illustrated in FIG. 2. This device has been found generally unsuitable for small vessel cardiovascular applications because it does not perform well in medium or low flow situations. Sizes below five millimeters have been found to be very difficult to make and securely implant inside patients. Furthermore, the Gianturco "Z" stent can only be expanded to one size which depends on the constrictive force applied against it by the vessel wall.
U.S. Pat. No. 4,922,905 to Strecker discloses a dilation catheter using a tube-like knitted structure. Before expansion, the individual meshes interlock loosely in a looped pattern. During radial expansion, the loops are deformed beyond the elastic limit of the filament material. In practice, it has been found that such woven stents use excessive material which may disturb the flow of blood through the stent and cause excessive tissue formation and thrombosis on the inside of the vessel. Furthermore, the woven design creates a highly flexible stent with limited loop strength which is undesireable for the reasons previously noted above.
U.S. Pat. Nos. 4,733,655, 4,739,762, and 4,776,337 to Palmaz discloses an expandable intraluminal graft. FIGS. 1A and 1B of Palmaz '665 illustrate a prosthesis made from diagonal elongate members which are stainless steel wires having a cylindrical cross section. The elongate members are preferably fixed to one another where they intersect in order to provide a relatively high resistance to radial collapse. The prosthesis is preferably made of continuous wire woven in a criss-crossed tubular pattern to form a wire mesh tube. This configuration, however, creates tines or points at each end of the criss-crossed pattern where the ends of the elongate members intersect. These tines can easily cause damage to the catheter balloon or inside wall of the vessel. Furthermore, the length of the tube will decrease as it is externally expanded which can cause further damage to the internal lining of the vessel.
FIGS. 1A and 1B of Palmaz '762 illustrate a thin-walled tubular member having a plurality of slots disposed substantially parallel to the longitudinal axis of the tubular member. The slots are uniformly and circumferentially spaced by connecting members. After expansion, the slots assume a substantially hexagonal configuration. The width of the slots may be substantially reduced so that the expanded slots assume the configuration of a parallelogram.
The Palmaz '762 design also has several drawbacks. First, the axial length of graft will shorten as it expands. Second, the slots create weak points near the connecting members which can break during expansion. Third, the design has been found to have limited expansion of about 4-6 times its unexpanded diameter and a metal surface area of about 10% of its expanded state. Finally, for small unexpanded diameters it can be difficult to form the slots in the tube. Consequently, the Palmaz '762 design offers limited capabilities for both large and small diameter vessels.
It has been found by the present inventor that an ideal vascular prosthesis should include several features. The stent should be formed from as little material as possible with a low profile (i.e. diameter) in its unexpanded state so that it can be inserted through the smallest possible hole in the vessel wall in order to control bleeding and damage to the vessel. A low profile also allows the stent to be more easily moved through narrow vessels. Furthermore, it is preferable that the unexpanded profile of the stent be independent of its expansion ratio. In other words, besides needing the smallest possible profile during insertion, there is also a need to be able to change the ultimate expansion ratio of the stent without affecting its unexpanded profile so that one size stent can be used with almost any size lumen.
The stent should also have high flexibility in its unexpanded state and excellent hoop strength in its expanded state. In practice, it has been found to be difficult to design a stent with both of these characteristics. Flexibility is needed to insert the stent through tortuous lumens while hoop strength is needed to resist the radial forces from the artery once the stent is in place. The stent should also be rigid once it is expanded inside a vessel in order to minimize its movement against the vessel intima after it is in place and to promote healing of the vessel after placement. Furthermore, the flexibility of the design should be adjustable without changing the size or configuration of the stent.
The stent should be atraumatic to vessels and blood cells. It should therefore be formed from as little biocompatible material as possible. The stent should not have any exterior tines or sharp edges which could damage the wall of the vessel. It should also not have any interior tines which could damage the catheter balloon or cause hemodynamic disturbances which might interfere with the flow of blood through the stent. The material from which the stent is formed is preferably a low memory, radio-opaque material. In other words, the stent should maintain its shape without recoil once it is expanded inside the vessel and should be visible during fluoroscopy in order to be able to verify that the stent has not migrated from its intended position.